The carb. on a 5100 can be finicky at times. I've had many that are difficult to get into adjustment. When adjusting the carb on these saws, the first thing to do is remove the limiter caps, screw both screws in until they seat lightly, and back both out 1 1/4 turns. Many times you can't do a complete adjustment with the factory settings. (The saws are tuned in Germany on Aspen fuel, and when using US fuel, they need to run a bit richer.) Turn the idle adjustment screw (S) in 2-3 turns and start the saw. Back out the S screw until the RPM is approx. 3200. Turn the L screw in until the RPM's start to rise, and continue turning until you hear them start to die off. Turn the L screw back out a touch. Readjust the S screw to obtain an RPM of 2900 to 3100. Pull the throttle and the saw should jump to life. If it hesitates, the mixture is too lean, back the L screw back out a touch more, and repeat the process until the saw does not hesitate any more. Realize that on this particular carb., adjustments as little as 1/16 of a turn on the low side can make a huge difference. Now do a "dump test". Allow the saw to sit in the normal position for 30 seconds. Abruptly tip the nose of the bar down and listen for a change in RPM. If the saw stalls, the L screw is out to far and needs to be adjusted back in a touch. Repeat the process until a "dump test" can be performed without the saw stalling. Recheck the acceleration, it should still be quick. If not, back the L screw back out a touch etc., etc., etc. It may take a few times back and forth, but if the carb. is functioning properly it will come into adjustment. Now run the saw at top-no-load (TNL) RPM. (Note: the chain should be tightened to the point in which is just starts to droop of the bar, and should spin freely. An over tightened chain will not yield an accurate adjustment.) Pull the throttle and observe the TNL RPM. Given that this saw has an electronic rev limiter, it will not exceed 14,500-14,700 RPM. Back the H screw out until you see the RPM's start to drop (richening the mixture). I have found that these saws perform extremely well with the TNL RPM right around 14,000. Surprisingly enough the H screw will be 1 3/4 to 2 1/4 turns out at this point. (The limiter caps are factory set to a max of about 1 1/2 turns.) Release the throttle, allow the saw to come to idle, pull the throttle again and it should still have a quick response. Recheck the TNL RPM and adjust as necessary, by turning the H screw out to decrease and in to increase. Finally recheck the idle, and tweak as necessary. Shut the saw off, and reinstall the limiter caps, making sure they are in the approx. center of their travel - this way fine adjustments can be made later without removing the limiter caps again.
Desired RPM's: Idle: 2900-3100 and TLN: 14,000-14,200
A little theory for those that don't understand the importance of performing the adjustments in this order: While idling, the carburetor only allows fuel into the engine through the low speed circuit. During high speed operation, most fuel comes through the high speed circuit, but a small amount may potentially be coming from the low speed circuit as well. Therefore, adjusting the low speed side first is critical in obtaining an accurate high speed setting.
